This invention pertains to computer apparatus and methodology for compressing compound original images of the type containing different visual component mixes of text, graphics, and photographic images, and the like. In particular, it relates to such apparatus and methodology which addresses the field of Mixed-Raster-Content (MRC)-based compression of compound images.
A preferred embodiment of, and manner of practicing, the invention feature: (a) the employment of a low-complexity, region-growing, segmentation algorithm to create a mask that defines, for subsequent compression, a differentiation, between foreground and background constituents (layers) of an original compound image; (b) the use of an embedded, topological, histogram-based boundary-encoding algorithm for compressing the defined foreground layer; and (c) an arbitrary-shape-wavelet-transform (ASWT)-based coding algorithm for compressing the defined background layer.
In offering improvement in the current state of the art regarding compound image compression, the present invention addresses the problem quite satisfactorily of optimizing the compression performance of MRC-based image encoding. As is well understood by those skilled in the art, due to the redundancy factors that are introduced by traditional MRC coding, wherein each pixel in an image has a corresponding value in multiple (such as three) layers, the matter of how to avoid information leakage becomes a major challenge. Previous approaches typically take what is known as a data-filling approach to solve issues raised by foreground and background image encoding, i.e., dealing with those masked pixels which are referred to as don't-care pixels, which masked pixels are replaced by certain values that maximize the smoothness of the overall image intensity field. The well understood data-filling solution in this situation enjoys simplicity because it transforms the problem of coding an arbitrarily shaped object into the issue of encoding a rectangular shape which has been well studied before. However, it is usually the case that any data-filling approach will not maximally eliminate the problem of redundancy, because theoretically no data bit should be used to code masked regions.
There also exist certain, more sophisticated, projection-based approaches for compressing partially masked images. In addition, the use of what is known as an arbitrary-shape-wavelet-transform (ASWT) has also been proposed in the past to address this problem.
By way of contrast, instead of following the prior art route of employing a data-filling-type approach, such as is traditionally thought of in conventional practice, the present invention addresses the issue of compressing partially masked images in a more explicit way, and in particular, in a way which exploits the arbitrary shape property of a masked image during the compression process. To deal with the foreground layer, the invention proposes a pallet-based, embedded, topological coding scheme. For the background layer, the invention extends the idea of employing a lifting scheme in cooperation with ASWT coding in order to improve and enhance that otherwise conventional prior art technique. It thus employs, for the background layer, what is referred to herein as a modified ASWT encoding, or compressing, procedure. The approach proposed by the present invention, as one will see on reading the detailed description below, does not suffer from any appreciable information leakage.
As will now be discussed in greater detail, in general terms, the method and apparatus of the present invention approach the issue of compressing a compound image of the type generally outlined above by applying a certain innovative collection of steps, which, as a collection, are unique in the art, and which, individually, include internal uniqueness per se. The first step performed, following appropriate review of the original compound image, is a segmentation step, wherein an original compound image, typically including some combination of text, and/or graphics, and/or photographic imagery, is divided into what is referred to herein as a foreground image layer (text and graphics), and a background image layer (photographic imagery), so as to create a mask relationship between these two segmented layers. The specific segmentation approach for mask creation involves analyzing an original image to locate regions, or neighborhoods, that contain relatively large and noticeable contiguous expanses of like-characteristic (such as like-color) pixels (for example, regions with a pixel population of about 8-pixels or more). Such regions are typically ones that have been created by a computer with a relatively limited pallet of colors, and these regions can be thought of as containing “pallet-based” images that have very distinctive and easily discernable boundaries between different colors.
The next steps involved in the practice of the invention relate to the application of compression algorithms per se, with a specifically different, layer-focused algorithm employed for each of the two segmented layers—foreground and background.
For compressing the foreground image, a technique is employed which is referred to as an embedded, topological, histogram-based boundary-encoding approach, wherein the specific boundaries that reside between adjacent regions of different pallet-based colors are encoded. All of the text and graphics regions which are appropriate for regional segmentation in accordance with the mask-creating step are so treated, and are considered to be part, therefore, of the foreground image layer. The background image layer is compressed utilizing an arbitrary-shape-wavelet-transform (ASWT)-based encoding algorithm which is applied in a somewhat modified and unique form, in the sense that a lifting scheme is imported into and utilized by this algorithm.
The resulting overall compression, therefore, of a compound image of the type now being discussed, is performed in a very computationally non-intensive manner, and indeed, in a relatively simple and very effective manner.
The various features and other advantages and new contributions to the relevant art that are made by the present invention will become readily apparent from a reading of the following detailed description of the invention taken along with a study of the accompanying drawings. Included in the detailed narrative which follows the description of the several drawings immediately below, is a section entitled “References”. This section lists a number of prior art technical references that provide helpful background relative to the features and the operation of the present invention. These references are identified with bracketed numbers, some of which also appear in the narrative text, where appropriate.